Flame resistant polyolefin compositions have been obtained heretofore by incorporating various additives. To be effective, an additive must promote suitable levels of flame resistance without causing inordinate sacrifices in the appearance, processing, mechanical properties, and economics of the flame resistant composition. Additives disclosed by the prior art include, halogenated organic compounds, and various combinations thereof. One such additive combination is described by Monroe in U.S. Pat. No. 3,216,960 (1965) wherein an organic pyrophosphate, a halogenated organic compound, and an inorganic additive are intimately mixed, in specified concentrations, with polypropylene to produce a flame resistant composition.
The prior art has also disclosed a flame resistant composition which is produced by mixing with polypropylene, an inorganic compound containing antimony and a halogenated organic compound in fixed proportions to each other and to the polypropylene. Such combinations have found wide use in flame resistant polypropylene compositions, in part because there is a synergistic action within such combinations. Said synergistic action is shown by a greater increase in oxygen index (hereinafter defined) when said combination is used than is achieved when the halogenated organic compounds or inorganic compounds are used alone. Antimony trioxide is commonly used as the inorganic component of such combinations. Examples of polyhalogenated organic compounds that are known to be useful in combination with an antimony compound so as to impart flame retardancy to resinous polyolefin compositions containing them are to be found in U.S. Pat. No. 3,275,596 (1966), U.S. Pat. No. 3,403,036 (1968), U.S. Pat. No. 3,419,518 (1968), and U.S. Pat. No. 3,738,958 (1973). These form part of a class to be later referred to as a flame retardant package containing an antimony compound such as Sb.sub.2 O.sub.3. Typical of such halogenated organic compounds are hexabromocyclododecane (HBCD), chlorinated paraffins, tetrabromophthalic anhydride (TBPA), and tetrabromoterephthalic acid (TBTA).
The greatest problem associated with the use of the aforesaid prior art additives involves the high additive concentrations necessary to effect suitable degrees of flame resistance. It is not uncommon for a flame resistant polypropylene composition to contain, by weight, 40% additives. In addition to the high cost of such flame resistant compositions, the use of high additive concentrations is generally accompanied by processing difficulties. For example, aliphatic and cycloaliphatic compounds containing bromine require lower extrusion temperatures (400.degree. F maximum) than for brominated aromatic compounds. Although aromatic compounds containing bromine can be processed at higher temperatures, they are less effective than aliphatic compounds as flame retardants. Chlorine containing additives tend to decompose, liberating hydrochloric acid which corrodes processing equipment.